Variable stroke piston-type and plunger-type positive displacement pumps are well known and have long been used in a variety of industries. A typical positive displacement pump will include at least one piston or plunger arranged to move in reciprocating fashion within a piston cylinder by means of a conventional crankshaft and connecting rod assembly. In a piston pump, the piston rod has a smaller diameter than the piston head, whereas in a plunger pump, the piston rod has the same diameter as the piston head. For illustrative purposes, the discussions herein are directed to piston pumps, although the principles apply to plunger pumps as well.
In operation, upon each suction stroke of the pump piston, a predetermined quantity of fluid is drawn into the piston cylinder depending upon the stroke length of the piston. During the pressure stroke of the piston, the fluid is discharged from the piston cylinder at a desired pressure. Regardless of the selected stroke length of the piston, a certain dead volume of fluid, known as the “unswept volume,” will remain within the piston cylinder because the piston does not completely evacuate the cylinder, even at the maximum stroke length of the piston. In most variable stroke pumping systems, the minimum unswept volume corresponds to the maximum stroke length of the piston, and the unswept volume increases as the stroke length of the piston decreases. When pumping compressible fluids or when pumping incompressible fluids at high pressures, the greater the unswept volume, the lower the efficiency of the pump due to compression of the fluid in the unswept volume area as well as expansion of the piston cylinder due to pressure. If the unswept volume becomes large enough and the pressure high enough, then all of the fluid just compresses and decompresses within the cylinder without actually leaving the pump. Therefore, a need exists for a variable stroke assembly capable of adjusting the stroke length of a pump piston while maintaining a substantially constant, and preferably minimized, unswept volume.
Variable stroke engine systems are also well known. A typical engine includes at least one piston arranged to move in reciprocating fashion within a piston cylinder, similar to a pump. However, the operation of an engine is opposite from the operation of a pump. In particular, for a 4-cycle engine, for example, the engine piston is extended during the exhaust cycle to a predetermined location within the piston cylinder, depending upon the selected stroke length of the piston. The engine piston is then retracted during the intake cycle while an air-fuel mixture is drawn into the piston cylinder through an inlet valve. The engine piston is extended again during the compression cycle to compress the air-fuel mixture. A spark plug is commonly used to ignite the fuel during the compression cycle, which increases the temperature and pressure within the cylinder. This heat and pressure act against the engine piston and cause it to retract during the power cycle at a given force, which is exerted on other engine components. Therefore, in contrast to a pump piston, which is retracted and extended by a force to draw fluid into the piston cylinder and then discharge the fluid at a higher pressure, an engine piston exerts a force during the power cycle to drive one or more engine components.
Regardless of the selected stroke length of the engine piston, a certain dead volume of fluid, i.e. the “unswept volume”, will always be present in the piston cylinder because the engine piston does not extend to the very end of the piston cylinder, even at its maximum stroke length. For proper engine operation, it is desirable to maintain a substantially constant compression ratio regardless of the stroke length of the piston. The compression ratio is the ratio of the total volume in the piston cylinder to the unswept volume in the piston cylinder. Therefore, a need exists for a variable stroke assembly capable of adjusting the stroke length of an engine piston while maintaining a substantially constant compression ratio.
The flow rate of a pump, or the power output of an engine, is a function of the speed at which the piston is driven, and the stroke length of the piston. Thus, to vary the flow rate of a pump, the speed of the motor that drives the pump may be varied, such as, for example, via a gear box, transmission, or variable speed drive. To vary the power output of an engine, the drive speed of the piston during the compression cycle may be varied. Alternatively, to vary the flow rate of a pump or the power output of an engine for a given drive speed, the piston stroke length may be adjusted by adjusting the distance that the piston is retracted and extended within the cylinder. Conventionally, the piston stroke length is adjusted manually via various mechanical means, such as, for example, by adjusting the throw of an eccentric lobe that rotates to drive the piston, or by adjusting swivels, cams, or linkages. Therefore, a need exists for an actuatable variable stroke assembly that enables adjustments to the stroke length of a pump piston or an engine piston, and which may also be automated for onsite or remote operation.